2658
Inorganic Chemistry, Vol. 9, No. 12, 1970 CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY, OF ALBERTA,EDMOSTOS, ALBERTA,CANADA UNIVERSITY
Kinetics and Mechanism of Oxidative Addition Reactions. I. Reactions of Methyl Iodide and Ethyl Iodide with a-CyclopentadienylcarbonylphosphineComplexes of Cobalt, Rhodium, and Iridium BY A. J. HART-DAVIS
AND
W. A. G. GRAHAM*
Received May 5 , 1970 A kinetic study of the oxidative addition reaction of .rr-CjHsM(CO)L (where M = Co, Rh, I r and L = tertiary phosphine) with CH31and CaHJ has been carried out. The reactions are first order in both metal complex and alkyl iodide. Rates are slightly greater for M = Ir than for M = Co or Rh, are much greater for CH3I than for CzHsI, and increase with increasing solvent polarity. The reactions are discussed in terms of the nucleophilicity of the metal atom.
Introduction Many reactions are known in which a molecule X-Y adds to a transition metal carbonyl compound in such a way that the metal becomes bonded to both X and Y and its formal oxidation state is increased by 2 . In the case of coordinatively unsaturated compounds, such as square-planar complexes of Ir(I)l,z and Rh(I), 8 , 4 addition of X-71 makes up the full complement of 18 electrons in the valence shell of the metal, e.g., for L = PCSH~(CH~)~~
In most other cases addition of X-Y is accompanied by elimination of a ligand such as a molecule of carbon m o n ~ x i d e ,e.g., ~ C & , C O ( C O ) P ( C ~ H ~ ) Iz ~ -+ CbH6COIzP(C&,)3$. co. The recently reported reactions between alkyl halides and the compounds CiHbM(C0)L (L = phosphine, M = Co, Rh,6aand IrGb),however, are oxidative additions which do not cause elimination of a ligand, although the starting materials are coordinatively saturated. In the case of cobalt and rhodium the products are acylmetal complexes, while with iridium ionic products are formed
+
CsHsCo(CO)P(CsHs)s
YCHa + CH3I +C5H~co-1 \
P(CeHs)3
We are investigating the kinetic, mechanistic, and preparative possibilities of these reactions. This paper
* T o whom correspondence should be addressed. (1) A. J. Deeming and B.L. Shaw, J . Chem. SOC. A , 1128 (1969). (2) P. B . Chock and J. Halpern, J . Amer. Chem. SOC.,88, 3511 (1966). (3) I. C. Douek and G. Wilkinson, J . Chem. SOC. A , 2604 (1969). (4) P. Uguagliati, A. Palazzi, G. Deganello, and U. Belluco, Inovg. Chem.,
9, 724 (1970). (5) R. B. King, ibid., 6, 82 (1966); R . Kummer and W.A. G . Graham, ibid., 7, 1208 (1968). (6) (a) A. J. Oliver and W. A. G. Graham, i b i d . , 9, 243 (1970); (b) A. J. Oliverand W. A. G. Graham, ;bid,, 9, 2653 (1970).
describes the effects of varying the metal, the phosphine, and the solvent on the rates of reaction with methyl iodide and ethyl iodide.
Experimental Section All operations were performed under an atmosphere of dry nitrogen. The cobalt complexes are particularly sensitive to air in solution. Solvents were fractionated twice under nitrogen before use. Tetrahydrofuran ( T H F ) was dried over sodium and benzophenone, toluene was dried over sodium, and acetone was dried over Drierite. hfethyl iodide and ethyl iodide mere fractionated under nitrogen before use. The phosphines were obtained from Strem Chemicals, Inc., Danvers, Mass., and from Alfa Inorganics, Inc., Beverley, Mass. Infrared data were obtained with a Perkin-Elmer Model 337 spectrophotometer, equipped with a Hewlett-Packard Model 7127A external recorder. The positions of the carbonyl stretching bands were determined by calibration with carbon monoxide in the terminal region and polystyrene in the acyl region and are 2 Nmr spectra were reprobably accurate to within 2 ~ ern-'. corded on a Varian A56/6OA instrument. Analytical data were obtained by the microanalytical laboratory of this department. Preparation of Starting Materials.-Preparation of CaHsIr(CO)P(CeHj)8 has been described.eb The complexes CJHLM(C0)PRa ( M = Co, R h ) were prepared by the following general m e t h ~ d . ~C E H ~ M ( C O(1-3 ) ~ g) and phosphine (1.0 mu1 per metal atom) were refluxed in hexane (40-80 ml) for a suitable period of time (see Table I ) . On cooling the reaction mixture complexes) )~ the product t o 0" (room temperature for the P ( C G H ~ separated, generally as large prisms. These were washed with hexane and recrystallized, when necessary, from CH%Clz-hexane. Some of the cobalt complexes (notably CSHBCO(CO)P(CGHII)S) crystallized from the reaction mixture in an analytically pure state. Preparation of Reaction Products.-The products of the oxidative addition reactions were prepared by the following general method. CsHsM(CO)L (100-500 mg) and R I 12 ml) were dissolved in CHzCll (3 ml) and kept a t room temperature for a suitable period of time (see Table I ) . Excess alkyl iodide and solvent were removed under vacuum and the product was recrystallized from CHzClz-hexane. This method gave good yields with CHJ of all products except CsH;CoI(COCHa)P(CeH1~)3, which we were unable even to observe in solution. The ethyl iodide products CjHBMI(COCzHe)P(CGHS)3 ( M = Co, R h ) were obtained with some difficulty, especially the cobalt compound, which decomposes almost as fast as it is formed. The analytical sample was obtained by three recrystallizations and hand picking of the best crystals. The ionic iridium products were produced in very high yield.eb (7) H. G. Schuster-Woldan and F. Basolo, J . Amer. Chem. SOC.,88, 1657 (1966).
T-CYCLOPENTADIENYLCARBONYLPHOSPHINE COMPLEXES
Inorganic Chemistry, Vol. 9, No. 12, 1970 2659
TABLE I PREPARATION, PROPERTIES, AND ANALYTICAL DATA Color
Dark red Dark red Dark red Dark red Orange Yelfow Dark red Dark red Dark red Red-brown Yellow Dark red Dark red Red-brown Yellow
Reaction time, hr
24 2 1 48 24
Yield,
%
c% calcd---.
MP,O ~C
86 65 60 62 88
153-156
f 1 0.3 0.3 1
83 60 80 70
100-110
170-175
f 48
...
66
f
Low
...
56
167-171
vc0,b em-1
1937 h c 1934 h 1934 h 1920 h 1957 h 1947 h 1655, 1628sh 1651, 1627 sh 1650, 1629 sh 1666, 1643 sh 2050 1646 1644 1660-1654 2043
foundH
7-%
C
H
C
69.6 64.8 57.9 66.6 62.9
4.9 5.2 5.6 8.9 4.4
69.3 64.8 58.1 66.6 63.0
5.0 5.3 5.5 8.9 4.6
54.0 48.6 41.7 50.0
4.2 4.3 4.4 3.9
53.8 48.8 41.6 50.0
4.2 4.2 4.3 4.1
54.8
4.4
...
54.0
4.5
..,
50.8
4.1
50.5
4.3
...
...
Obtained on Kofler hot stage. Rhodium compounds melted with decomposition. Cobalt compounds decomposed over a wide temperature range. h = hexane. Other spectra taken in CHzC12. 1919 cm-' in CHzClz; 1918 cm-l in Nujol. Nrnr in CDCls, T values: C&, 2.5 (multiplet); CsHs, 5.23 (singlet); CHs, 6.63 (singlet). e Nmr in CDCb, 7 values: C6H.5, 2.5 (multiplet); C5H5,4.76 See ref 6b. (singlet); CHa, 7.08 (singlet). a
25
20
-
15
c
A. L = P P h M e 2
B. L = P P h 2 M e
'ti
%
C. LXPPh,
h
2
D..L = P(CdHI,)3
Y0
d
E!
10
5
OO
1.2
Figure 1.-Second-order rate plots for the reactions of CsH&o(CO)L with CHaI in dichloromethane a t 25", showing the effect of different phosphines.
2660 Inorganic Chemistry, VoZ. 9,No. 12,1970
A. J. HART-DAVIS AND W. A. G. GRAHAM
Kinetic Measurements.-The observed rate constants recorded below refer to the rate of disappearance of starting material, as determined by the intensity of its carbonyl stretching band in the infrared spectrum. All reactions were carried out under pseudo-first-order conditions, using a t least a tenfold excess of alkyl iodide. Concentrations of metal complexes from 0.003 M (0.5-mm ir cells) up to an optimum of 0.03 M (0.2-mm ir cells) were used in such a way as to maintain the necessary excess of alkyl iodide. All reactions went to completion. Beer's law was showp to be obeyed over the concentration range used for CaHsRh(CO)P(C6Hj)s in CHzClz and was assumed t o be obeyed by the other complex-solvent combinations studied. Reaction flasks were kept in a constant-temperature bath and small samples were withdrawn a t suitable intervals for infrared measurement. Between 13 and 20 readings were generally taken over a period of about 3 half-lives of the reaction. Plots were drawn of In log ( T , / T ) , where T i s the transmittance of the peak being followed and TOis the transmittance of the base line a t that point, determined a t "infinite" time. The linearity
TABLE IV RATESO F REACTIOX O F CaHaRh(CO)P(CsH&)a A N D CjHjIr(CO)P(C&)s with CH31 CsHsRh(C0) P(CaHa)a, CHzClz, 2.50 [CHsIl, lO4kobsd, 103k2, -44 sec-1 .VI-1 sec-1
0.0 0.164 0.310 0.428 0.522
0.0004 5.3 10.8 14.8 18.2
lO4kobsd,
sec-1
loski, M - 1
sec-1
15" 0.378 5.3 (4.7a) 0.424 6 . 2 0.806 11.6 1.397 18.4
1.4 1.5 1.4 1.3
20 O 0.333 6 . 9 8.2 0.403 0.863 1 6 . 3 1.047 19.9
2.1 2.0 1.9 1.9
[CHaI], M
0.114 0.190 0.191 0.233 0.396 0.539 0.738 0.820
1O4kobsd,
sec-1
25 2.50 5.30 5.08(5.5") 6.48 10.9 14.0 19.6 20.3
sec-1
2.2b
2.8 2.7 2.8 2.8 2.6 2.7 2.5
30 O 0.190 8.4 4.4 25" 0.228 9 . 2 4.0 0.462 18.2 0.0 0.007 ... 3.9 0.107 2 . 4 1 2.3b 0.482 19.1 4.0 Low results for kl a Rate of growth of product acyl band. were generally obtained a t low [CHJ] , which probably reflects evaporation of CH3I from the reaction mixtures. TABLE 111 RATESO F REACTION O F C&CO(CO)L WITH CHaI AT 25". EFFECTSO F VARIATION I N PHOSPHINE A N D SOLVENT L = P(CaHa)s, solvent T H F [CHsI], 104k0bsd, lOikz, M sec-1 M-1 sec-1
0.574 1.434 1.971
3.09 8.0 12.5
5.4 5.6 6.3
L=P(C#Ha)a,solvent (CHa)zCO [CHsI], l o 4 k & s d , 103kz, M sec-1 M - 1 sec-1
0.188 0.324 0.500 0.769
4.3 7.3 10.9 17.1
2.3 2.3 2.2 2.2
L=P(CaHs)a, solvent CHsCh1O3k2, [CHaI], lo'kobad, M sec-1 M - 1 sec-1
0.163 0.344 0.498 a
Ca. 13" Ca. 19" Ca 3 P
Ca. 8 Ca.6 Ca.6
L = P(CaHdaCHz, solvent CHzCla [CHaI], lo'kobsd, 102k2, A4 sec-1 M - 1 sec-1
0.038 0.068 0.084 0,150
5.6 9.1 10.9 23.0
1.5 1.3 1.3 1.5
L = P(CnHs)(CHs)z, solvent CHaClz [CHsI], 1O4kobsd, l O ~ k ~ , A4 sec-1 M-1 sec-1
0.042 0.050 0.076
13.8 14.9 22.8
3.3 3.0 3.0
L = P(CsHii)s, solvent CHzClz [CHsI], 1o4kobsd, 1O4k2, M sec-1 M-1 sec-1
0.0 0.005 0.306 1.87 1.022 6.06 2.810 14.3 Decomposition caused poor kinetics in CHICN.
... 6.1 5.9 5.1
...
0.057 0.070 0.096 0.164
3.3 3.5 3.5 3.5
[CHd], M
1O4kobsd,
0.227 0.366 0.656 0.795
3.0 4.8 8.8 10.6
sec-1
1O4kobsd,
102kz,
M
sec-1
sec-1
-1
1.4c 1.8~ 2.6 2.3
8.1 12.6 (11.7") 25 37
CsHsIr(CO)P(GWs)a, CHICIZ, 100
100
[CHsIl, AI
1O3kz, -44-1 sec-1
1.3 1.3 1.3 1.3
0.063 0.098 0.142
Io'kubsd,
sec-1
102kz,
.If
5 . 7 (5.Qa) 9.8 14.6
-1
sec-l
0.91 0.99 1.02
CsHaRh(CO)P(CsHa)s, toluene, 250
[CHaIl, M
101k2, M-1
2.50
[CHaII, M
CsHaRh(CO)P(CaHs)a, CHSClz,
TABLE I1 RATESOF REACTIONS OF C ~ H ~ C O ( C O ) P ( C ~ H E ) ~ WITH CHII I N CHnClz [CHsI], M
CsHsIr(CO)P(CsHs)s, CHZCIZ,
1O4kobsdl
sec-I
lOsk?, M - 1
sec-1
0.288 0.22 1.212 1.27 1.705 1.90 3.283 5.1 a Rate of growth of product band (2050 c1n-l). * S e e text and Figure 2. See footnote b , Table I1 These reactions are too fast to obtain precise kinetic data by our method. TABLE V RATESOF REACTION WITH C?HJ
IN
CH2Cl2AT 25"
~ C ~ H ~ C O ( C O ) P ( C ~ H ~ ) ~ - 7--CsHsRh(CO)P(CsHs)s---[CzHaIl, 10akobsd, 106kz, [CzHsIl, 106k,bsd, 106kz, M sec-1 M -1 sec-1 M sec-1 M - 1 sec-1
1.21 2.38
---
7.5 14.5
6.2 6.1
CsHsCo(CO)P(CsHs)aCHs-7 1osk0bsd, lO'ka, [CaHaIl, M sec-1 -11-1 sec-1
0.703 2.3 3.3 0.716 2.3(2.3a) 3 . 3 1.084 3.1 2.8 1.262 3.5(3.2") 2.8 1.603 4.8 3.0 2.399 7.0 2.9 a Rate of growth of product band
0.778 1.562 2.162 3.048
2.6 4.9 7.7 8.7
3.4 3.1 3.6 2.8
---CsHsIr(CO) P(CaH6)a - - 7 [CzHsIl, 106koasd, lOak?, *w sec-1 J - 1 sec-1
0.386 0.890 3.765
0.86 1.84 6.5
2.2 2.1 1.7
of these plots demonstrates that the reactions are first order with respect to the metal complex, the pseudo-first-order rate constants being given by the negative gradients. The rate constants were evaluated by a least-squares method and generally showed standard errors of 1--370. The rates of the reactions were not noticeably affected by wrapping the flasks in aluminum foil. Plots of observed rate constants against alkyl iodide concentrations gave good straight lines with near-zero intercepts (Figure 1). Second-order rate constants were evaluated from the gradients of these lines and are collected in Table VI. "In) are listed in dividual" second-order rate constants ( h o b a d / [KX] Tables 11-V. The reactions of C:HaRh(CO)P(CsHj)a with CH3I in toluene are exceptional and are discussed below. The cobalt starting materials used were unstab1e.j The solids darkened with time, and solutions, while remaining clear, showed growth of bands corresponding in position and relative intensity to those of C5H~Co(CO)2. This reaction was rapid for a few minutes, during which time some 37, of the starting material disappeared, and then became very slow. Kinetic
T-CYCLOPENTADIENYLCARBONYLPHOSPHINE COMPLEXES SECOND-ORDER RATECONSTANTS (AT 25’)
Inorganic Chemistry, Vol. 9, No. 12, 1970 2661
TABLE VI REACTIONS O F C&M(CO)L
FOR THE
--
Solvent
P(CsHa)s
CH3I CHzC12 25.7 CzHsI CHiClz 0.061 ... CHaI C6HsCHs CHaI THF 6 CHaI (CHa)zCO 22.2 Ca. 60 CHaI CHiCN a All values are quoted as 104k2( A 4 - I sec-l).
M = Cn PCaHr(CHa)i P(CeHa)zCHa
150 0.28
...
studies in the case of the P(C&)3 and P(CBHII)Scomplexes showed the rate of this reaction to be negligible compared with the rates of all but the slowest of the oxidative additions studied. Further it was found that in the case of CsHsCo(CO)P(Ce“)a the side reaction could be suppressed by the addition of triphenylphosphine (1 mol per Co atom) and that this added P(C&6)3 had no appreciable effect on the observed rate cohstant of the CH31 addition. Thus the occurrence of the side reactions cannot seriously interfere with the kinetic studies. The side reactions of C ~ H ~ C O ( C Owere ) L increasingly extensive in the order L = P(CP,HII)~< PCd-h(CHa)z < P(C6Hs)zCHa < P(C&)3 and in the solvents CH3CN < (CH3)zCO < T H F < CHzClz < toluene < hexane, suggesting that an initial equilibrium dissociation of phosphine was involved. The compounds CsHgRh(CO)P(C6H5)8 and C&Ir(CO)P(C6H6)3 were much more stable, the rhodium compound decomposing a t least 10 times more slowly than the cobalt analog. The cobalt-acyl products were somewhat unstable in coordinating solvents. Thus in T H F the acyl CO band grew and then decayed again, and greenish yellow crystals of an apparently ionic product were formed. In CHsCN no acyl band could even be observed in solution, while the expected product CsH6CoI(COCH~)P(C~H seemed I ~ ) ~ to be unstable even in CHZCI~.
Results Most of the kinetic reaction products were prepared and characterized by means of analysis (Table I). The positions of the carbonyl infrared bands of these compounds correspond closely to those of the kinetic reaction mixtures a t infinite time. The observed rate constants (kobsd) and the secondorder rate constants derived from them ( K z ) are listed in Tables 11-V and k z values are collected in Table VI. Activation parameters are listed in Table VII.
WITH
--.--.
CHsI
AND
c - L
=
P(CeHn)a
Rh
306
5.5
...
...
36 0.030 0.71
...
...
CzHsIa P(CeHda-M-
7
Ir
210 0.20
*..
the activated complex. In fact the pattern of the kinetics is similar to that found for the addition of methyl iodide to square-planar Rh(I)314 and Ir(I)2 complexes and for Menschutkin reactions between amines and alkyl halides.s’s The reaction a t the alkyl iodide is most probably a nucleophilic attack on the a carbon or an electrophilic attack on iodine. Since the carbon atom in carbonyl ligands is often reactive toward base, lo it seems unlikely that in this case i t should act as a bucleophile. Furthermore the electron density a t the metal atom is high in these CbHSM(C0)L complexes, as shown by the low frequency of the carbonyl stretching band. It seems likely that the metal atoms have some nucleophilic power; the Lewis basicity of transition metals has recently been reviewed. l 1 We therefore propose that the rate-determining step in these reactions is a nucleophilic attack by the metal atom on the a carbon of the alkyl iodide. This produces an ionic species,0& which is stable in the case of iridium. With cobalt and rhodium, however, the ionic species is only an intermediate which collapses in a rapid second step involving coordination of iodide ion and migration of the alkyl group to the carbonyl ligand (carbon monoxide “insertion”). ki slow
&
L r
TABLE VI1 ACTIVATION PARAMETERS FOR REACTIONS WITH CHsI IN CHzClz AH*, kcal/mol
CsH6CO(CO)P(C&b)a 12 (*l) 10 (fl) CsHsRh(CO)P(CaHs)a C ~ H ~ I ~ ( C O ) P ( C ~ & ) S Ca. 8-9” a Approximate values.
AS*? eu
-31 (*3) -36 ( k 3 )
The proposed first step may be represented as
- (35-4O)o
Discussion The second-order nature of these reactions and the observed negative entropies of activation are consistent with a bimolecular mechanism. The acceptable agreement between the rate of disappearance of starting material and the rate of formation of product shows that there is only one rate-determining step in each reaction. The marked effect of solvent on the reaction rate suggests that there is considerable charge separation in
L
L
J
(8) K. B. Wiberg, “Physical Organic Chemistry,” Wiley, New York, N. Y . , 1964. (9) E. S.Gould, “Mechanism and Structure in Organic Chemistry,” Holt, Rinehart and Winston, New York, N. Y . , 1959. (IO) E. 0. Fischer and A. Massbol, Angew. Chem., ‘76, 645 (1964). (11) J. C. Kotz and D. G. Pedrotty, OvganomeloL Chem. Rev., Sect. A , 4, 479 (1969).
2662 Inorganic Chemistry, Vol. 9,No. 12, 1970
A. J. HART-DAVIS AND W. A. G. GRAHAM
The "latent" lone pair of electrons in the starting material is intended to have a nucleophilic but not a stereochemical significance. Similar nucleophilic attacks by the metal atoms have been proposed in the oxidative additions of methyl iodide to complexes (Ar3P),M(CO)X (M = Rh,3 Ir2) and (C6H5)3PRh(C0)2CI. Furthermore in the latter cases the rhodium but not the iridium complexes undergo methyl migration to the carbonyl ligand. There is clearly much similarity between these systems and our own. There is also a close relationship between the proposed stages of this reaction and related reactions of species with one more negative charge,6ae.g., M ~ I ( C O ) ~ CHII + CH3Mn(C0)6 I- and CH3Mn(C0)5 I- --c IMn(C0)4COCH3.Support for our proposed mechanism comes from isolation of one of the ionic intermediates. The compound C5H5RhBr(COCH3)PC6H5(CH3)2 reacts rapidly with N ~ B ( C ~ HinBmethanol )~ to give a high yield of the white ionic product [C5H5Rh(CH3)COPc6H5(CH3)2]+B(C&)4- ( Y C O 2068 cm-1).6a The reaction between C S H S R(COCH3) ~I P (C~HE,)~ and NaB (C6H5)4 is complicated by the low solubility of the former in methanol, but by using a CH2C12-CH30Hmixture about 3-4 mg of pale yellow crystalline product was isolated. This had YCO 2070 cm-l in CH2C12. On addition of (CH3)dNI this band disappeared rapidly and was replaced by the expected bandsI2 a t 1666 and 1643 cm-I of the product CbHsRhI (COCH3)P (C6H5)3. Effect of Solvent.-The reaction rates are strongly dependent on the solvent. The reaction between C5H5Co(CO)P(CeH5)3 and CH3I can only be studied in a limited range of solvents because of the instability of starting materials and products, but relative rates a t 25" were found: THF, 0.2; (CH&CO, 0.9; CH2C12, 1.0; CH3CN,2-3 (approximate). Study of the reaction between C5H5Rh(CO)P(C$H& and CHJ in toluene was complicated by the coincidence of the carbonyl stretching band with a toluene mode and by slow crystallization of the product. The second-order rate plot (Figure 2a) shows an upward curvature which is probably a medium effect; a t high concentrations of methyl iodide (up to 0.3 mol fraction) the polarity of the medium is much greater than that of pure toluene. A plot of kOb,d/[CH3I] against [CH,I] (Figure 2b) was extrapolated back to obtain the second-order rate constant in pure toluene. This is the value recorded in Table IV and is 50 times smaller than the corresponding rate constant for CH2C12. Effect of Alkyl Group and Phosphine.-Ethyl iodide reacts 400-1200 times more slowly than methyl iodide with the C5HsM(CO)Lcompounds investigated. This large rate difference and also the rate differences produced by different phosphines in the CjHbCo(C0)L compounds are probably caused mainly by steric effects. Ingold has surveyedlg an extensive series of nucleo-
+
+
P
0
1
0
s
+
(12) The occurrence of two acyl bands in molecules of this type has been noted previously and ascribed to a conformational effect.88 (13) C. K. Ingold, Quaul. Rev., Chem. Soc., 11, 1 (1957).
I
0
05
,
1
10
I
15
20
[Mer]
(MI-+
25
30
3.5
Figure 2.-Reactions of CaH,Rh(CO)P(C&Is)s with CHd in toluene, showing the effect of increased polarity caused by high CH31concentrations.
philic displacements on alkyl halides by halide ions (Finkelstein reactions) in acetone a t 25". The slower rates of all the ethyl halides than those of their methyl analogs are shown to be predominantly a steric rather than an electronic effect, and this steric effect is further shown to be largely ponderal in origin. Brown has studied the effect of steric hindrance on nucleophilic displacements on alkyl halides, and we may compare some of his results14 on substituted pyridines with our data on C5H5Co(CO)Lnucleophiles. Relative rates a t 25' for nucleophilic displacements on methyl iodide are as f ~ l l o u ~ s ' ~ CH(CHJ2
(3 / /
/P(C&h C~H~CO
CH,CHJ
65
6.7
3.2
,P(C& '
C~H~CO
\CO
\CO
1.0
6.1
e:
12
CHB
/pc6H5(CH3)2 C5H5Co
\co 10.7
In both series of nucleophiles changes are being made in the bulkiness of the groups some three atoms removed from the reaction site, and the effects on the rate are of the same order of magnitude in both cases. The increasingly basic P(C8H5)2CH3and PCoHs(CH& ligands might be expected to make the corresponding C5HsCo(CO)L complexes increasingly nucleo(14) H. C . Brown, J . Chem. Soc., 1249 (1956). (15) Nitrobenzene was the solvent for the pyridine reactions and dichloromethane for those of the cobalt compound. (16) Actual value of ki = 2.45 X 10-6 M-1 sec-1.14
Inorganic Chemistry, VoZ. 9, No. 12, 1970 2663
COBALT( III)-N-METHYL-(3) -ALANINATO COMPLEXES
philic with respect to C ~ H ~ C O ( C O ) P ( C B H The S ) ~ . ~ ~similar process for the three metals Co, Rh, and Ir in the compounds C6H6M(CO)P(C6H6)3. The kinetic tricyclohexylphosphine complex, however, shows that patterns and activation parameters (for CH31) are all the steric effect is important, since P(CeH11)3 is one of of the same kind. The failure of the iridium reaction to the most basic but also one of the bulkiest phosundergo the second stage is interesting; a related case p h i n e ~ . ~As expected, the CO stretching band in has been discussed. CSHBCo(CO)P(C~HII)B is appreciably lower in freThe relative rates of reaction of the complexes with quency than those of the other complexes used (Table different metals are as follows: C&I: Co, 1.0; Rh, I). We were unable to isolate any acyl product, but the rate of disappearance of C~H&O(CO)P(C~HII)~1.4; Ir, ca. 8; C & , I : Co, 2; Rh, 1; Ir, 6. Thus the with CHaI was one-fifth as large as that of C~HSCO- expectedg increase in nucleophilicity on descending the group is apparent in the reaction with methyl iodide but (CO)P(CsH6)3,so that in this case a t least the steric is outweighed in the ethyl iodide case by the alternating effect predominates. The rate difference between CH3I and CzHJ is effect so common in this triad.? C&Rh(CO)P(CsH6)3 has a higher CO stretching frequency than either the Co ~ surprisingly large. Hine stated that “in S Nreactions or the Ir analogs, which suggests that the electron methyl halides are usually found to be 4-150 times as density a t the metal atom is relatively low and may be reactive as the corresponding ethyl halides.”18 The related to its low nucleophilicity. Similar trends in absolute values of our rate constants, however, are not reactivity have been noted for metal carbonyl anions. 1 9 unlike those reported for well-known systems. I 3 , l 4 In order to gain further insight into these systems we The complexes under investigation behave therefore as have studied the reactions of C S H S R ~ ( C O ) P ( C ~ H ~ ) ~ nucleophiles of moderate reactivity but unusual steric with benzyl and allyl halides. These results will be selectivity. reported in a subsequent paper. Effect of Metal.-The initial nucleophilic attack on methyl and ethyl iodides seems to be an essentially Acknowledgment.-We thank the National Research Council of Canada for financial support. (17) A.J. Deeming and B. L. Shaw, J. Chem. SOC.A , 1802 (1969). (18) J. Hine, “Physical Organic Chemistry,” McGraw-Hill, New York, N.Y.,1956.
(19) R. E. Dessy, R. L. Pohl, and R. B. King, J. Amer. Chem. SOC.,88, 5121 (lQ66).
CONTRIBUTION FROM THE RESEARCH SCHOOL OF CHEMISTRY, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A. C.T. 2600, AUSTRALIA
Preparation, Stereochemistry, and Conformational Analysis of Gobal t (II I )-Trie th ylenet e tramine-N-methyl- (8)-alaninato Complexes BY D. A. BUCKINGBAM, I. E. MAXWELL,
AND
A. M. SARGESON*
Received May 29, 1970 I
The preparations and structural assignments of some &-Co(trien)(N-Me-(S)-ala)2+ions are described; the A( +)a9-P~-(SSR)Co(trien)( N-Me- (S)-ala) *, .A( )u&- (RSS)-Co(trien) (&Me-(S)-ala) +, A( - )w&- (RRS)-Co(trien) (N-Me-( S)-ala)2 +, and A( )sss-Pz-(RRS)-Co(trien)(N-Me-(R)-ala)2+ diastereoisomers have been separated by fractional crystallization and resolution methods or observed following mutarotation a t pH 7 or 12. Under equilibrium conditions, p H 6.5 ( 2 5 O ) , the A-RRS(S)and A-RSS(S) isomers exist in approximately equal amounts; a t p H 12 (25’) the equilibrated solution contains A-RRS(R) ( ~ 6 0 % ) ,A-RRS(S) (-20%), and A-RSS(S) (-20%). Strain energy minimization calculations support the structural assignments and predict that significant angular deformations will occur in the molecular ions to relieve steric crowding.
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Introduction Previous experiments have shown that the reaction of P-Co(trien) (OH) (HzO) + (trien = triethylenetetramine) with (S)-proline is kinetically controlled and that equal amounts of the diastereoisomers A-pz-(RRS)Co(trien) (@)-pro) + and A-Pz- (SSS)-Co(trien)((S)pro)2+are formed.lr2 However, it was not possible to
* T o whom correspondence should be addressed. (1) D. A. Buckingham, L. G. Marzilli, I. E. Maxwell, A. M. Sargeson, and H. C. Freeman, Chem. Commun., 583 (1969). (2) Nomenclature: R a n d S designate the asymmetry about the “angular” and “planar” asymmetric N atoms of triethylenetetramine and the secondary N atom of the amino acid in t h a t order and follow the rules suggested by C. K.Ingold, V. Prelog, and R. S. Cahn, Angew. Chem., Int. Ed.
measure the relative stabilities of the A-&(RRS)Co(trien) ((S)-pro)2f and A-L%-(RRR)-Co(trien)((R)pro) 2 + ions, since base-catalyzed dissociation of the amino acid moiety preceded mutarotation. The slow rate of mutarotation compared to other chelated amino acids3 was attributed to the requirement that protons Engl., 6, 385 (1966),and accepted by the IUPAC. For consistency we have also used this nomenclature to specify the configuration of the u-C atom of the amino acid (e&, (S)-proline = L-proline). The use of 8, 81, a n d Pzfollows that used by L. G. Marzilli and D. A. Buckingham, Inorg. Chem., 6 , 1042 (1967). The absolute configuration about the cobalt center is indicated by the prefix A or A: ibid., 9,1 (1970). (3) D. A. Buckingham, L. G. Marzilli, and A. M. Sargeson, J. Amer. Chem. Soc., 89, 5133 (1967).
